Catalytic exhaust gas purifiers are often used in motor vehicles in order to purify the exhaust gases from the vehicle's engine. However, present catalytic converters (catalyzers) are not able to operate at full purification efficiency in connection with cold-starting of a vehicle. This problem can be overcome by arranging an additional, electrically-heatable start catalyzer in front of the ordinary catalyzer. By heating up the start catalyzer electrically, the warm-up time for the ordinary catalyzer can be shortened, which results in improved purification efficiency during cold starts. Such a catalyzer arrangement is shown in International Patent No. WO 80/10470.
A problem which occurs in connection with an electrically-heatable start catalyzer is that it requires a powerful electrical supply in order to be activated. This can, for example, be achieved by arranging an additional battery in the vehicle, which is however unnecessarily expensive and complicated. It is therefore desirable to have a catalyzer arrangement which is not dependent on an electrical supply, but which still produces a shortening of the main catalyzer's "ignition" time (the so-called "light-off" time), i.e. the time which elapses from starting the engine until the catalyzer is working at optimal purification efficiency. This should, in such a case, result in improved purification efficiency of the entire catalyzer arrangement.
In connection with conventional catalyzers, there is an additional problem in that the incoming exhaust gases impinge directly onto the catalyzer. This produces a turbulent flow immediately in front of the catalyzer, which leads to inefficient operation of the catalyzer as well as a comparatively high back-pressure, which is a disadvantage compounding the reduction in the efficiency of the catalyzer. One way of solving this problem is to direct the exhaust gases at an angle to the catalyzer, for instance in accordance with the disclosure in European Patent No. 420,462. This arrangement, however, presents a disadvantage in that it has relatively poor flow characteristics, since not only the inlet but also the outlet are angled with respect to the catalyzer.
A main object of the present invention is to solve the aforementioned problems and provide an improved arrangement for catalytic exhaust gas purification, in particular for shortening the light-off time of such an arrangement.
An additional object of the present invention is to achieve a correct measurement of the conversion efficiency of the aforementioned catalyzer arrangement, in order to provide information for a diagnosis of its operation. This allows an indication, e.g. in the form of a warning light, which warns the vehicle driver that the catalyzer is defective and has to be replaced.